The present invention relates to temperature stable monolithic ceramic capacitors with base metal electrodes, and more particularly to such capacitors having a plural phase ceramic dielectric material.
In a patent to Prokopowicz, U.S. Pat. No. 3,523,028 issued Aug. 4, 1970 and having a common assignee with this invention, there is described a somewhat similar ceramic material being characterized as consisting of two phases. However, the ceramic dielectric material employed in this invention is capable of being fired together with interleaved base metal electrodes and differs in formulation and structure from the patented material; and the two materials are considered to have complimentary uses.
In a pending U.S. patent application by Eror et al, Ser. No. 130,692 filed Apr. 2, 1971 now U.S. Pat. No. 3,920,781 issued Nov. 1, 1975 and a corresponding British patent No. 1,333,698 issued Oct. 10, 1973, there is described a monolithic ceramic capacitor having base-metal electrodes. The single phase large grain ceramic in these capacitors has a dielectric constant of around 10,000 and a temperature coefficient of capacitance (TCC) better than +22% and -56% from +10.degree. C to 85.degree. C, which TCC limits correspond to the Electronic Industries Association (E.I.A.) standard, designated Z5U. Another E.I.A. standard, designated X7R requires temperature stability within .+-. 15% from -55.degree. C to 125.degree. C. A large majority of all monolithic ceramic capacitors made today are required to meet one or the other of these two standards.
The increasingly high cost of the noble metals commonly used as electrodes in monolithic capacitors intensifies the need for a low cost base-metal electrode counterpart. Since it is necessary to fire base metal electrode capacitors in a reduced oxygen atmosphere to avoid oxidizing the electrode, severe restraints are imposed on the selection of the constituents of the ceramic material which are subject to being reduced and made semiconducting at firing, and which could therefore produce low insulation resistance in the completed capacitor. Attempts to make similar capacitors based on barium titanate with base-metal electrodes have resulted heretofore in dielectrics with substantially lower permittivities and/or temperature stability over a narrower range of temperature. In these previous attempts, large quantities of additives have been included in the ceramic that form low permittivity phases and as a consequence have resulted in low permittivities, (generally less than 2000) and/or inadequate temperature stability.
It is therefore an object of this invention to provide a monolithic ceramic capacitor having base metal electrodes that meets the TCC standard designated X7R.
It is a further object of this invention to provide a temperature stable monolithic ceramic capacitor having a ceramic dielectric of which the dielectric constant is greater than 2000.
It a further object of this invention to provide a temperature stable monolithic ceramic capacitor having a high capacity per unit volume and exhibiting an insulation resistance-capacitance product that is greater than 500 ohm-farads at 25.degree. C and greater than 15.degree. at 125.degree. C.